There is growing acceptance of nonhuman primate (NHP) models and studies as playing a critical part in HIV-1 basic vaccine discovery, potentially providing valuable information on the immunogenicity and efficacy of vaccine concepts, advancing candidate vaccines into human clinical trials. As it is unlikely that a single NHP model for HIV-1 disease will satisfy all the preclinical needs, efforts are needed to improve or optimize existing models so that they can be utilized effectively, as well as develop new experimental tools to broaden the applications of NHP models. With this in mind, we recently find that intravenous (IV) or intrarectal (IR) inoculation of Indian rhesus macaques (RMs) with the late R5 SHIVSF162P3N isolate recapitulates key pathogenic features of HIV-1 infection in humans including acute CD4+ T cell depletion in the gut, uncontrolled replication, and progression to AIDS with switch in coreceptor preference. The consistency of high setpoint viremia and pathogenicity seen in SHIVSF162P3N IV and IR infected macaques, and the finding that the envelope of SHIVSF162P3N displays antigenic properties typical of the transmitted founder HIV-1 R5 viruses lead us to hypothesize that this virus will be highly relevant and useful for advancing the discovery of HIV-1 candidate vaccines that stimulate both effector arms of the immune system, allowing for comparative as well as synergistic studies of T and B cell-based vaccine concepts. To further optimize this new virus, we will assess the impact of dose (single high vs. repeat low), route of inoculation (intravaginal vs. intrarectal), and macaque origin (Indian vs. Chinese) on R5 SHIVSF162P3N pathophysiology, with the goal of establishing the breadth of its utility for vaccine research (Aim 1). Attempts will also be made to generate a pathogenic R5 SHIVSF162P3N molecular clone to study the mechanisms of viral escape and breakthrough in vaccinated macaques, potentially providing insights on the immunological correlates of protection (Aim 2). Lastly, to address the issue of viral diversity and more closely mimic the situations of HIV-1 exposures in humans, a novel approach is proposed whereby the envelope of neutralization-sensitive subtype C R5 Envs will be used in the construction of SHIVs for serial passaging in RMs, the objective being the generation of pathogenic non-subtype B R5 SHIVs as challenge viruses in NHPs (Aim 3). These studies to optimize/refine existing R5 SHIVSF162P3N models and develop additional experimental tools such as pathogenic, CCR5-using SHIVs that carry genetically-diverse envelopes as challenge virus stocks should provide a wider range of biologically relevant models and challenge viruses for preclinical evaluation of HIV-1 vaccines, improving the predictive value of NHP studies. The development of prevention strategies such as vaccines remains a major priority for the control of the AIDS pandemic. Studies in nonhuman primates (NHP) are anticipated to play a critical part in HIV-1 basic vaccine discovery, providing valuable information on the comparative immunogenicity and efficacy of vectors and immunization strategies, advancing promising candidate vaccines into human clinical trials. Research efforts to establish a set of biologically relevant and extensively-characterized models should broaden the applications, and improve the predictive value of NHP studies.